Bradley M. Oborn

The sophistication and diversity of radiation medicine modalities continue to increase significantly in both cancer imaging and cancer radiation therapy. With over 50% of cancer patients receiving some form of radiation procedure as part of their cancer management plan, the importance and impact of quantitative radiation detector systems for quality assurance in radiation medicine should not be underestimated. There are numerous challenges faced by silicon‐based detector systems used for dosimetry in radiation therapy that are discussed in this review article, especially considering emerging radiation therapy modalities that incorporate ultra‐high irradiation dose‐rates (e.g., MRT, FLASH‐RT, VHEE‐RT), while others utilize mixed radiation fields (e.g., proton‐RT, heavy ion therapy) or diffusion‐based dose delivery techniques (e.g., Alpha DaRT). Such challenges create exciting opportunities for new radiation detector materials. This concise review paper explores the role of silicon in radiation detection applications and highlights the challenges it faces in achieving optimal performance. Additionally, this review discusses new materials that are emerging as strong candidates for the next generation of radiation detectors, emphasizing both the opportunities and challenges associated with these materials.

Purpose: To evaluate the accuracy of different dosimeters and the treatment planning system (TPS) for assessing the skin dose due to the electron streaming effect (ESE) on a 1.5 T magnetic resonance (MR)‐linac.Method: Skin dose due to the ESE on an MR‐linac (Unity, Elekta) was investigated using a solid water phantom rotated 45° in the x‐y plane (IEC61217) and centered at the isocenter. The phantom was irradiated with 1 × 1, 3 × 3, 5 × 5, 10 × 10, and 22 × 22 cm² fields, gantry at 90°. Out‐of‐field doses (OFDs) deposited by electron streams generated at the entry and exit surface of the angled phantom were measured on the surface of solid water slabs placed ±20.0 cm from the isocenter along the x‐direction. A high‐resolution MOSkin™ detector served as a benchmark due to its shallower depth of measurement that matches the International Commission on Radiological Protection (ICRP) recommended depth for skin dose assessment (0.07 mm). MOSkin™ doses were compared to EBT3 film, OSLDs, a diamond detector, and the TPS where the experimental setup was modeled using two separate calculation parameters settings: a 0.1 cm dose grid with 0.2% statistical uncertainty (0.1 cm, 0.2%) and a 0.2 cm dose grid with 3.0% statistical uncertainty (0.2 cm, 3.0%).Results: OSLD, film, the 0.1 cm, 0.2%, and 0.2 cm, 3.0% TPS ESE doses, underestimated skin doses measured by the MOSkin™ by as much as –75.3%, –7.0%, –24.7%, and –41.9%, respectively. Film results were most similar to MOSkin™ skin dose measurements.Conclusions: These results show that electron streams can deposit significant doses outside the primary field and that dosimeter choice and TPS calculation settings greatly influence the reported readings. Due to the steep dose gradient of the ESE, EBT3 film remains the choice for accurate skin dose assessment in this challenging environment.

Purpose: The aim of in vivo skin dosimetry was to measure the absorbed dose to the skin during radiotherapy, when treatment planning calculations cannot be relied on. It is of particularly importance in hypo‐fractionated stereotactic modalities, where excessive dose can lead to severe skin toxicity. Currently, commercial diodes for such applications are with water equivalent depths ranging from 0.5 to 0.8 mm. In this study, we investigate a new detector for skin dosimetry based on a silicon epitaxial diode, referred to as the skin diode.Method: The skin diode is manufactured on a thin epitaxial layer and packaged using the “drop‐in” technology. It was characterized in terms of percentage depth dose, dose linearity, and dose rate dependence, and benchmarked against the Attix ionization chamber. The response of the skin diode in the build‐up region of the percentage depth dose (PDD) curve of a 6 MV clinical photon beam was investigated. Geant4 radiation transport simulations were used to model the PDD in order to estimate the water equivalent measurement depth (WED) of the skin diode. Measured output factors using the skin diode were compared with the MOSkin detector and EBT3 film at 10 cm depth and at surface at isocenter of a water equivalent phantom. The intrinsic angular response of the skin diode was also quantified in charge particle equilibrium conditions (CPE) and at the surface of a solid water phantom. Finally, the radiation hardness of the skin diode up to an accumulated dose of 80 kGy using photons from a Co‐60 gamma source was evaluated.Results: The PDD curve measured with the skin diode was within 0.5% agreement of the equivalent Geant4 simulated curve. When placed at the phantom surface, the WED of the skin diode was estimated to be 0.075 ± 0.005 mm from Geant4 simulations and was confirmed using the response of a corrected Attix ionization chamber placed at water equivalent depth of 0.075 mm, with the measurement agreement to within 0.3%. The output factor measurements at 10 cm depth were within 2% of those measured with film and the MOSkin detector down to a field size of 2 × 2 cm². The dose–response for all detector samples was linear and with a repeatability within 0.2%. The skin diode intrinsic angular response showed a maximum deviation of 8% at 90 degrees and from 0 to 60 degree is less than 5%. The radiation sensitivity reduced by 25% after an accumulated dose of 20 kGy but after was found to stabilize. At 60 kGy total accumulated dose the response was within 2% of that measured at 20 kGy total accumulated dose.Conclusions: This work characterizes an innovative detector for in vivo and real‐time skin dose measurements that is based on an epitaxial silicon diode combined with the Centre for Medical Radiation Physics (CMRP) “drop‐in” packaging technology. The skin diode proved to have a water equivalent depth of measurement of 0.075 ± 0.005 mm and the ability to measure doses accurately relative to reference detectors.

With the recent clinical implementation of real‐time MRI‐guided x‐ray beam therapy (MRXT), attention is turning to the concept of combining real‐time MRI guidance with proton beam therapy; MRI‐guided proton beam therapy (MRPT). MRI guidance for proton beam therapy is expected to offer a compelling improvement to the current treatment workflow which is warranted arguably more than for x‐ray beam therapy. This argument is born out of the fact that proton therapy toxicity outcomes are similar to that of the most advanced IMRT treatments, despite being a fundamentally superior particle for cancer treatment.In this Future of Medical Physics article, we describe the various software and hardware aspects of potential MRPT systems and the corresponding treatment workflow. Significant software developments, particularly focused around adaptive MRI‐based planning will be required. The magnetic interaction between the MRI and the proton beamline components will be a key area of focus. For example, the modeling and potential redesign of a magnetically compatible gantry to allow for beam delivery from multiple angles towards a patient located within the bore of an MRI scanner. Further to this, the accuracy of pencil beam scanning and beam monitoring in the presence of an MRI fringe field will require modeling, testing, and potential further development to ensure that the highly targeted radiotherapy is maintained.Looking forward we envisage a clear and accelerated path for hardware development, leveraging from lessons learnt from MRXT development. Within few years, simple prototype systems will likely exist, and in a decade, we could envisage coupled systems with integrated gantries. Such milestones will be key in the development of a more efficient, more accurate, and more successful form of proton beam therapy for many common cancer sites.

The new era of intra-fraction dose tracking in radiation therapy delivery demands new dosimetry methods, whereby a moving frame of reference as a function of time may be required. This introduces a new paradigm into radiation therapy dose verification. The term we propose to describe this is dynamic dosimaging, which by our definition is tracking the location of a dosimeter array in real time during on-line radiation dose acquisition.